I understand that this isn't exactly Seti@home science, but I think that the people on this forum are qualified to answer better than most other places.

My question is, is there such thing as a largest or smallest wavelength? Or do radio waves extend out to the size of the universe, and gamma rays shrink to undetectable sizes?

We have DC which is in effect a wave of infinite size, so we know there is no limit going this direction, except perhaps the age of the Universe if it is to be generated inside the Universe.

On the other end the plank length might be considered a limit. Such a wave would pack a lot of energy. At some point going smaller the wave/photon would consist of the entire energy of the universe and I assume that might be a real physical limit.

We have DC which is in effect a wave of infinite size, so we know there is no limit going this direction, except perhaps the age of the Universe if it is to be generated inside the Universe.

On the other end the plank length might be considered a limit. Such a wave would pack a lot of energy. At some point going smaller the wave/photon would consist of the entire energy of the universe and I assume that might be a real physical limit.

Sorry, but what is DC? And with the Planck length, isn't it simply that it is impossible to probe anything smaller, or is it that things actually can't get any smaller? The limits on the energy you can put into it makes sense though.
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Ryan,
The smallest electromagnetic waves we know about are gamma rays. In theory, there could be smaller waves but currently we can't detect them. Also, something has to transmit the wave in the first place, so gamma radiation is emitted when particles go through radioactive decay.

On the large scale, theoretically electromagnetic waves could be colossal, the size of the universe. But in practise, again you need something to transmit the wave. So being practical, stars could emit waves that are the diameter of the star. Maybe in theory, but only in theory, a whole galaxy could act as one big transmitter and transmit one massive electromagnetic wave.

Those would be the theoretical upper and lower limits to alternating electromagnetic waves.

My question is, is there such thing as a largest or smallest wavelength? Or do radio waves extend out to the size of the universe, and gamma rays shrink to undetectable sizes?

I think this is a very subtle question. Mathematically, their is no limit to wavelength. But physically, the largest and shortest extremes transcend science into philosophy, it seems. One might claim that the largest dimension is the size of the universe. If that is the 'longest' wavelength, the energy of such a wave is nearly zero. But does the idea of a wave make sense if the temporal coherence is lost? That is, if such a wave could be measured at some point in the universe, thereby transferring its energy from the wave to the measurement device, when would the wave at the other end of the universe collapse? Wouldn't information about the measurement would have to travel the extent of the wave and back? It seems like a relativistic and quantum conundrum to me.

At the shortest extreme, remember that a wave possesses energy in proportion to its inverse wavelength. If the amount of energy in the universe were to be infinite, then we could have an infintely small wavelength. But that is unlikely (at least philosophically), and I suspect most people might prefer to think of the universe's energy being large but finite. In this case the shortest meaningful wavelength, assuming all matter in the universe were to be translated to energy, would be determined by this value of energy. But that seems silly, for several reasons. First, a universe consisting of only energy is meaningless; we and no one else would be around to measure it (or care). Such a propagating wave would have nowhere to go (perhaps). And so on. And, quantum mechanically, we have the vacuum energy to consider; the zero point energy of the vacuum. The precise value of the wavelength would be quantum mechanically uncertain due to these quantum fluctuations.

There are so many other things to consider not mentioned above. Gee I hate Physics; it is so confusing. But I suspect a more reasonable response would be what folks have brought up below, based on empiricism (Simple DC limit to Gamma rays; ignore the diameter of the star idea because it seems incorrect).

The two suns are impacted by a gravitational wave at the same instant that runs parallel to them

The resonate and produce an electromagnetic wave between them

This wave propagates outward and eventually runs into another star and another and another, etc

Each encounter causes a slight resonace or dampens the original wave. The E Wave is sometimes amplified and sometimes dampened still propagating way from the two star source. At the speed of light that wave will encounter vast distances of nada out there and most eventually will dissapate as the energy is entropied away in time..

Occasionally the wave will interact and gain energy mosly it will dissapate.

So these colossal EM waves are rare.. Their origin speculative... Their lifespan finite due to entropy. What might cause two stellar cores many light years apart to resonate in coincidence? I speculate a gravitational wave but thus far they remain thoretical..

One might look a a swiming pool and drop one drop of water on its still surface. The wavve propagates outward but will eventually be so weak as to be undetectable. Now drop 1000 drops on its surface evenly distributed and the cacaophony of wavelets created will be immense. Some of those waves will netralize one another some will combine into larger wavelets. The universe is like that a cacaphony of differing incoherent waveforms being produced non-uniformly in multiple dimensions and actually in multiple time coherences.

So the answer to large is above the answer to small is a matter of what can begin the oscillation of EM at what subatomic level...
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Never engage stupid people at their level, they then have the home court advantage.....